Magnetic resonance imaging (“MRI”) is used primarily in medical settings to is produce high quality images of the inside of the human body. MRI is based on the principles of nuclear magnetic resonance (“NMR”), a spectroscopic technique used by scientists to obtain information about the microscopic chemical and physical characteristics of molecules. MRI started out as a tomographic imaging technique, producing an image of the NMR signal as a thin slice through the human body. MRI has now advanced beyond tomographic imaging to become a volume imaging technique.
The MRI image is created when the nuclei of atoms are placed in a magnetic field and are exposed to non-ionizing radio frequency (“RF”) energy at a specific frequency. An RF pulse is emitted that excites the nuclei away from equilibrium.
When the pulse is switched off, the nuclei return to their original state and in the process emit energy at RF frequencies. The signal is picked up by a receiver coil and is converted into images through the application of a sophisticated mathematical algorithm
An MRI system comprises a main magnet, a gradient system. RF coils, a transmitter, a receiver and a computer equipped with imaging software. The magnets in an MRI system may be superconducting magnets, electromagnets or permanent magnets.
A permanent magnet system differs from superconducting magnet systems in that superconducting systems require significant amounts of electricity and liquid helium for maintaining coolant circulation, air conditioning systems, and for the various electronics components. The site requirements for superconducting systems are also more demanding than for permanent magnet systems due to the specific needs of the cryogen.
Electromagnets also require cooling (usually by water) and produce a relatively weak magnetic field.
Strong magnetic fields, low eddy currents and a high degree of homogeneity of the magnetic field are critical to producing good quality MRI images. Most prior art MRI systems using permanent magnets are limited in the strength of the magnetic fields they can produce having regard to the magnetic materials generally available to produce the magnets.
Japanese Patent 08045729 to Sumitomo Special Metals Co. Ltd., dated Feb. 16, 1996 discloses a magnetic field generating device for an MRI incorporating a C-shaped yoke.
Chinese Patent 94115507.2 to Dong et al., dated Apr. 24, 1996 discloses an Open-C shaped permanent magnet design. The production model of the magnet was made with poles made of NdFeB. While this material has very good magnetic properties, it is very expensive. As such it is not commonly used for large permanent magnets. The cost of the NdFeB used in the Dong et al. design remains high.
The Dong et al. design uses octagonal poles, pole pieces and rings. However the octagonal shapes produce regions of high flux density at the corners of the octagons, which distorts the shape of the field in the imaging volume (the gap between the poles). Ideally, the horizontal cross-section of the field in the gap is perfectly circular. It is not possible to achieve this with the octagonal design without careful shimming to correct the field distribution after the magnet is to manufacture. The Dong et al. design is therefore limited in the homogeneity of magnetic field it can achieve, and suffers the disadvantage of requiring elaborate shimming and having high eddy currents.
The Dong et al. design also uses a stepped shape of corner pieces of magnetic material (steel) located at the inside corners of the “C”. This was done to decrease the amount of metal and thus reduce the weight and cost of the magnet. However, this results in some areas having too much material removed and some too little. This causes saturation in the steel and increases the amount of leakage flux and the size of the field away from the magnet. For safety reasons, this requires a larger room.
The outer edges of the pole piece and rings on the Doug et al. magnet extend vertically from the pole. The function of these pieces is to improve the uniformity of the field in the gap, but if improperly designed, they reduce the efficiency of the magnet thus requiring a larger than necessary amount of NdFeB to be used to obtain the desired field strength.
In addition, a significant amount of time is needed to shim the Dong et al. magnet after it is manufactured. The Dong et al. pole pieces were made of steel which is desirable to improve the homogeneity of the field, however it has very low resistively which allows eddy currents to form when an MRI scan is being performed. Eddy currents reduce the signal to noise ratio of the system, resulting in poorer images. The Dong et al. magnet was created out of a number of steel plates that had to be machined to the proper dimensions, and bolted together. This increased the cost of each magnet significantly. Furthermore, since the yoke is bolted together, there is an increased risk that the magnetic force between the poles of the magnet could be stronger than the bolt strength holding the yoke arms parallel. To eliminate this risk, vertical columns needed to be added between the yoke arms as additional support members. This partially obstructed the gap between the poles causing difficulties in positioning the patient bed in the gap during scans.
It is an object of the present invention to provide an Open-C shaped, NdFeB-based permanent magnet design that minimizes the amount of NdFeB required, but that nonetheless provides good homogeneity of field in the gap, minimizes the need for shimming and minimizes eddy currents.